In the following the expression absolute orientation refers to how the vehicle is directed relative to a system of axes fixed relative to the ground.
In many application fields it is important to get an accurate estimation of the relative orientation of a vehicle. One such application field is production of maps when images can be taken from airborne vehicles such as unmanned airborne vehicles, often abbreviated UAV. Other application fields are as components in navigation equipments for aeroplanes, helicopters and UAVs or included in missiles such as cruise missiles.
There are quite a number of different ways known to estimate the absolute orientation. One way of doing this is to observe landmarks and compare with a map. It is also possible to combine an inertial motion unit, IMU, that can comprise accelerometers and gyros, with a magneto-meter. Another way is to combine an IMU with a GPS. Examples of rather complex navigation methods using camera, GPS and IMU are known from WO 2005100915 A1, U.S. Pat. No. 5,894,323 A and WO 9735166 A1. Another example is disclosed in US 20060018642 disclosing an infra red camera and GPS used in combination with laser distance measuring and magnetic compass. These by references exemplified methods are component requiring and complex in the treatments of obtained information.
Accordingly, these mentioned methods have different advantages and drawbacks differing in under which circumstances they can be used, the accuracy of the results obtained, time delay introduced and so on.